The present invention relates to a helper drive apparatus for a turbine, in which an AC motor is coupled with the turbine and a load is driven by both, at a desired power-sharing ratio.
In general, a large-scale compressor is driven at a constant speed by a steam turbine. If the capacity of such a compressor is increased, equipment of the turbine must be replaced by larger one.
Due to recent low electric power costs during nighttime, motor drive tends to be more economical than turbine drive during the nighttime operation.
For this reason, a helper motor system has been proposed. This system utilizes an AC motor, which is advantageous from the viewpoint of high-speed operation. This motor is coupled to a turbine, and its output is controlled, by means of a power converter, so as to remain at a predetermined value. Thus, the load driven by the turbine is shared partially with the AC motor.
With the helper motor system, the turbine is subjected to speed control, based on a speed reference, by a mechanical governor, while the AC motor is subjected to power control based on a power reference.
This being so, the shaft speed of the turbine, the load machine, and the AC motor, respectively, are controlled by the speed control of the turbine, while power is supplied to the AC motor in accordance with the power reference set separately from the speed reference. Therefore, if the relationship between the speed references of the turbine and motor is not managed with due care, the system will likely break down.
For example, if the speed reference of the turbine is decreased while the power reference of the AC motor remains constant, power to be supplied to the load may become transiently short. Further, when an operation is to be performed steadily at a point of maximum economy or of highest efficiency, the optimal point must then be re-adjusted in accordance with the current operating condition of the load machine. Such an adjustment is cumbersome and time-consuming.
Since, in the case of the conventional helper motor system, the turbine system includes the mechanical operation, the system's control response is slow. Consequently, the response of the turbine, with respect to output, is slow when an abrupt change in load occurs, resulting in a large variation in the speed of the turbine shaft. When the speed control of such a system is performed by a motor system, no mechanical operation is involved, and thus, a quick response can be obtained. However, this system requires the use of a special motor having an output capacity which can cover a wide range of variations in load over a long period of time. Thus, the conventional helper motor system is disadvantageous in terms of practical equipment investment.